1. Field of the Invention
The present invention relates to a columnar structure film, a method of manufacturing same, a piezoelectric element having a piezoelectric film composed of a columnar structure film, a liquid ejection apparatus using same, and a piezoelectric ultrasonic oscillator.
2. Description of the Related Art
Piezoelectric elements are used in applications such as piezoelectric actuators mounted in inkjet recording heads. A piezoelectric element has a piezoelectric body that expands and contracts with increase and decrease in the intensity of an applied electric field, and electrodes through which the electric field is applied to the piezoelectric body. Known piezoelectric materials are lead zirconate titanate (PZT), or substituted PZT in which a portion of the A site and/or B site of the PZT is substituted with another element.
Taking account of the compactification and reduction in thickness of piezoelectric elements, it is desirable that the piezoelectric body is composed of a thin film. For example, such the thin film can be a monocrystalline film, which has high voltage tolerance and high durability since there are no grain boundaries in the monocrystalline film. However, with monocrystalline films, the film formation rate is extremely slow, productivity is low, and formation over a large surface area is difficult, which means that practical applicability is poor. On the other hand, in a case where a piezoelectric body is constituted of a thin film having a polycrystalline structure, the piezoelectric body has voltage tolerance lower than cases where a bulk member having a large film thickness is used, and if driven repeatedly by applying voltage, there is a tendency for the insulation to break down and deterioration of displacement occurs readily.
A known film structure for a piezoelectric film having a polycrystalline structure is a columnar film structure constituted of a plurality of column-shaped bodies which extend in directions non-parallel to a substrate surface. Japanese Patent Application Publication Nos. 2000-307163 and 2007-335779 disclose layered structures of varying grain size in piezoelectric films having columnar film structures.
Japanese Patent Application Publication No. 2000-307163 discloses that a structure is formed in such a manner that the grain boundaries in the piezoelectric film are discontinuous in the film thickness direction. It is also disclosed that a piezoelectric film has crystalline structures layered in the film thickness direction, and the layers are formed in such a manner that the grain sizes in adjacent layers are mutually different. It is described that the composition reduces leakage paths and thereby improves voltage tolerance (paragraph 0049, for example). The layered structure is obtained by means of a sol gelation method (paragraphs 0024 to 0033, for example); more specifically, by repeating, a plurality of times under different conditions, a step of applying a liquid containing metal alkoxides including the constituent metals of the piezoelectric body onto a substrate, and subsequently carrying out heat treatment. In the layered structure, the grain size in the first layer is 3000 nm to 4000 nm, the grain size in the second layer is 50 nm to 80 nm, and the grain size in the third layer is 100 nm to 500 nm (paragraphs 0028, 0030 and 0031, for example).
Japanese Patent Application Publication No. 2007-335779 discloses a piezoelectric film having a layered structure constituted of a layer containing a larger amount of lead than the stoichiometric proportion and a layer containing a smaller amount of lead than the stoichiometric proportion, and describes that the leakage paths due to lead dioxide are shut off and the durability is improved by means of this composition (paragraphs 0019 to 0020, for example). It is also described that the number of occurrences of leakage paths can be reduced by increasing the grain size in the layer having a large amount of lead (paragraph 0036, for example). The piezoelectric film having the layered structure is formed by sputtering (paragraphs 0042 and 0065, for example). The piezoelectric film has a ratio of “grain size in film with large amount of lead”/“grain size in film with small amount of lead”=2.0 (paragraphs 0045 and 0068, for example).
Both Japanese Patent Application Publication Nos. 2000-307163 and 2007-335779 propose piezoelectric films having layered structures with varying grain sizes, but do not specifically investigate how far the grain size should be varied.
In Japanese Patent Application Publication No. 2000-307163, the layered structure described above is obtained by a sol gelation method which repeats, a plurality of times under different conditions, a step of applying a liquid containing metal alkoxides including the constituent metals of the piezoelectric body onto a substrate, and subsequently carrying out heat treatment. In this method, the process is complicated and it takes time to form the piezoelectric film. It is also difficult to form the piezoelectric film to a small thickness.